# Power Electronics Questions and Answers – 3-Phase Bridge Inverters-1

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “3-Phase Bridge Inverters-1”.

1. Identify the below given circuit.

a) Three-phase bridge regulator
b) Three-phase bridge type semi-converter circuit
c) Three-phase bridge thyristor inverter
d) Three-phase bridge IGBT inverter

Explanation: It is a three-phase bridge type inverter. As SCRs are used as the switching device, it is called as a thyristor inverter.

2. A three-phase bridge inverter requires minimum of _____________ switching devices.
a) 3
b) 4
c) 6
d) 8

Explanation: It requires a min. 6 devices, 2 in each leg. Switching devices could be anything BJT, MOSFET or an IGBT. SCRs are used when very high power ratings are required.

3. The below given inverter circuit is a __ step inverter.

a) 3
b) 2
c) 6
d) none of the mentioned

Explanation: The three-phase bridge type is a 6-step inverter. That means, the firing changes from one SCR to another 6 times per cycle.
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4. In the three-phase bridge inverter, each step consists of
a) 30°
b) 60°
c) 90°
d) will depend on the value of the firing angle

Explanation: It is a 6 step inverter. Hence, 360/6 = 60°. This means that the SCRs are gated every 60° in proper sequence.

5. In inverters, to make the supply voltage constant
a) an inductor is placed in series with the load
b) capacitor is connected in parallel to the load side
c) capacitor is connected in parallel to the supply side
d) none of the mentioned

Explanation: A large C connected across the input terminal keep the supply voltage from altering.

6. In the 180° mode VSI, ___________ devices conduct at a time.
a) 5
b) 2
c) 3
d) 4

Explanation: Three devices conduct at a time. One from the upper pair and two from the lower pair or vice-versa.

7. In the figure given below, for 180° mode of operation if T1 is fired at 0°. Then SCRs T3 and T5 should be fired at _________ and _________ respectively.

a) 180°, 360°
b) 90°, 180°
c) 120°, 240°
d) none of the mentioned

Explanation: T1-T4 form the first pair. T3-T6 form the second pair, and like-wise. For the 180° mode, each SCR conducts for 180°, but the groups of SCRs lag the prior group by an angle of 120°. e.g. If T1 is fired at 0 then T3 must be fired at an angle of 0 + 120° and T5 at 120 + 120 = 240°.

8. For a three phase bridge inverter in the 180° mode, ___________ devices are conducting from 120° to 180°.

a) T1, T6, T5
b) T2, T6, T5
c) T1, T6, T5
d) T1, T2, T3

Explanation:

 Group I T1 T1 T1 T4 T4 T4 T1 T4 Group II T6 T6 T3 T3 T3 T6 T6 T6 Group III T5 T2 T2 T2 T5 T5 T5 T2 Step No I II III IV V VI I II

Each step consists of 60°. 120° to 180° will be step III.

9. _________ SCRs conduct from 300° to 360°.

a) T1, T2, T3
b) T4, T5, T6
c) T4, T3, T2
d) T1, T6, T5

Explanation:

 Group I T1 T1 T1 T4 T4 T4 T1 T4 Group II T6 T6 T3 T3 T3 T6 T6 T6 Group III T5 T2 T2 T2 T5 T5 T5 T2 Step No I II III IV V VI I II

Each step consists of 60°. 300° to 360° will be step VI.

10. The diodes D4 and D1 will conduct from

a) they will never conduct
b) 300° to 360°
c) 120° to 180°
d) insufficient information

Explanation: This will depend on the nature on the load, which is not mentioned in the above problem.

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